November 8, 2005
Granites Continued, Rapakivi Texture
Bowen’s argument: granites are produced solely by fractional crystallization because they
all plot near the ternary minimum of qtz, plag, and kspar in composition
Problem: this system becomes eutectic-like if pressure PH2O increases or if Ca added to
mixture in anorthite, CaAl2Si2O8 Æ therefore, you can ALSO make granite by partial
melting, since the 1st melt will be at the eutectic composition no matter what the
composition is
In a system with a ternary minimum, the first melt is variable. In a system with a
eutectic, the first melt is at the eutectic. The last material produced by fractional
crystallization has a composition near the minimum or the eutectic, depending on the
system.
Bowen was right that fractional crystallization can create granite from a mafic parent
magma (basalt). He was wrong that this was the only possible mechanism. The proof is
rapakivi texture.
Rapakivi texture has kspar crystals ringed with plagioclase. Anti-rapakivi texture has
plagioclase crystals ringed with kspar. Overgrowth textures such as these are evidence of
fractional crystallization and a record the order by which the mineral components reached
the solidus. In the system Ab-Kspar-An-Qtz-H2O (Ab and An are both types of white
plagioclase), we can clearly show that anti-rapakivi texture can be achieved by fractional
crystallization. It is, however, impossible to crystallize kspar followed by plagioclase,
rapakivi texture, by fractional crystallization.
To make rapakivi:
Take liquid A in equilibrium with plagioclase and kspar solid phases. Decompress. Now
A is in the plag primary phase volume, so plag crystallizes to return the liquid to
equilibrium. This forms plag rims on all the crystals, including kspar.
Q. How would we decompress our magma suddenly?
A. Huge silicic eruptic events – ash flow welded tuffs, as in the San Juans mountains (a
6000 cubic km eruption) and at Yellowstone (3 eruptions in 2.4 million years, 2 of them
> 1000 cubic km eruptions). The magma bursts open the cap of the volcano, which then
sinks to make a caldera. An eruption such as this catastrophically decompresses the
magma. Rapakivi textures may record large volume catastrophic decompressions of
silicic magma!
Layered intrusions – mafic – source of platinum for mining – appear preserved in crust
The big ones are from the Proterozoic and the Archean, such as the 2.05 billion year old
Bushfeld. >90% of the Pt in the world comes from one layer of the Bushfeld that extends
for hundreds of km in South Africa.